[Alnusjaponica]

Summary

Replace the XxxStorageOptions/new_with_option(s) construction pattern with builders following the API style of std::thread::Builder: settings are configured by chaining setter methods and the storage is created with build. As with thread::Builder::new(), XxxBuilder::new() is the only entry point (consistent with PedAnovaBuilder in #262).

let storage = InMemoryStorageBuilder::new()
    .apply_discard(true)
    .attr_format(AttrFormat::Json)
    .build();

let storage = SQLite3StorageBuilder::new(file_path)
    .apply_discard(true)
    .build()?;   // Result

let storage = JournalStorageBuilder::new(Box::new(backend))
    .apply_discard(true)
    .build()?;   // Result

Changes

• Add InMemoryStorageBuilder (with Default), SQLite3StorageBuilder, and JournalStorageBuilder, exposing new(), chainable setters (apply_discard, attr_format), and build()
• Remove InMemoryStorageOptions, SQLite3StorageOptions, JournalStorageOptions and new_with_option/new_with_options
• Keep new on InMemoryStorage/SQLite3Storage/JournalStorage with the existing signatures; it now delegates to the builders
• Update the pyo3 bindings (including ToRustStorage's cache) to construct storages through the builders
• Update a doc comment in cache.rs that referenced the removed options structs

Verification

• cargo test --locked --all-features — all pass (including new doctests)
• cargo test --locked -- --ignored (from rustuna_storage, with Optuna installed) — 6 pass
• cargo clippy --locked --workspace --lib --bins --tests --examples --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• Python: uv run --no-sync pytest tests/ — 706 passed, 16 skipped

🤖 Generated with Claude Code

[c-bata]

@Alnusjaponica Is this PR ready for review?